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Federal Register / Vol. 86, No. 165 / Monday, August 30, 2021 / Rules and Regulations Because of the limitations in most monitoring studies, EPAs standard approach is to use water exposure models as the primary means to estimate pesticide exposure levels in drinking water. Modeling is a useful tool for characterizing vulnerable sites and can be used to estimate upper-end pesticide water concentrations from infrequent, large rain events. EPAs computer models use detailed information on soil properties, crop characteristics, and weather patterns to estimate water concentrations in vulnerable locations where the pesticide could be used according to its label Ref.
24 at 2728. EPAs models calculate estimated water concentrations of pesticides using laboratory data that describe how fast the pesticide breaks down to other chemicals and how it moves in the environment at these vulnerable locations. The modeling provides an estimate of pesticide concentrations in ground water and surface water. Depending on the modeling algorithm e.g., surface water modeling scenarios, daily concentrations can be estimated continuously over long periods of time, and for places that are of most interest for any particular pesticide.
EPA relies on models it has developed for estimating pesticide concentrations in both surface water and groundwater.
The most common model used to conduct drinking water assessments is the Pesticide in Water Calculator PWC.
PWC couples the Pesticide Root Zone Model PRZM and Variable Volume Water Model VVWM models together to simulate pesticide fate and transport from the field of application to an adjacent reservoir. Ref. 24 at 2728.
The PWC estimates pesticide concentrations for an index reservoir that is modeled for site-specific scenarios i.e., weather and soil data in different areas of the country. A detailed description of the models routinely used for exposure assessment is available from the EPA OPP Aquatic Models website: https www.epa.gov/pesticidescience-and-assessing-pesticide-risks/
models-pesticide-riskassessmentaquatic.
In modeling potential surface water concentrations, EPA attempts to model areas of the country that are vulnerable to surface water contamination rather than simply model typical concentrations occurring across the nation. Consequently, EPA models exposures occurring in small highly agricultural watersheds in different growing areas throughout the country, over a 30-year period. The scenarios are designed to capture residue levels in drinking water from reservoirs with
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small watersheds with a large percentage of land use in agricultural production. EPA believes these assessments are likely reflective of a small subset of the watersheds across the country that maintain drinking water reservoirs, representing a drinking water source generally considered to be more vulnerable to frequent high concentrations of pesticides than most locations that could be used for crop production.
When monitoring data meet certain data quantity criteria, EPA has tools available to quantify the uncertainty in available monitoring data such that it can be used quantitively to estimate pesticide concentrations in drinking water. Ref. 25 Furthermore, monitoring data can be used in a weight of evidence approach with model estimated concentrations to increase confidence in the conclusions of a drinking water assessment.
b. Drinking Water Level of Comparison DWLOC
The drinking water level of comparison DWLOC is a benchmark that can be used to guide refinements of the drinking water assessment DWA.
This value relates to the concept of the risk cup, which EPA developed to facilitate risk refinement when considering aggregate human health risk to a pesticide. Ref. 26. The risk cup is the total exposure allowed for a pesticide considering its toxicity and required safety factors. The risk cup is equal to the maximum safe exposure for the duration and population being considered. Exposures exceeding the risk cup are of potential concern. There are risk cups for each pertinent duration of exposure e.g., acute, short-term, chronic. The exposure durations most commonly of interest for acute or shortterm pesticide exposure risk assessments are 1-day, 4-day, and 21day averages. For example, the relevant exposure duration for AChE reversible inhibition from exposure to carbamate insecticides is 1-day, while AChE
irreversible inhibition resulting from exposure to OP insecticides is usually 21-days based on steady-state kinetics.
Ref. 19
In practice, EPA calculates the total exposure from food consumption and residential or other non-occupational exposures and subtracts this value from the maximum safe exposure level. The resulting value is the allowable remaining exposure without the potential for adverse health effect.
Knowing this allowable remaining exposure and the water consumption for each population subgroup e.g., infants, the Agency can calculate the DWLOC,
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which is the estimate of safe concentrations of pesticides in drinking water. Using this process of DWLOC
calculation allows EPA to determine a target maximum safe drinking water concentration, thereby identifying instances where drinking water estimates require refinement. Ref. 24 at 1920.
c. Scale of Drinking Water Assessment Although food is distributed nationally, and residue values are therefore not expected to vary substantially throughout the country, drinking water is locally derived and concentrations of pesticides in source water fluctuate over time and location for a variety of reasons. Pesticide residues in water fluctuate daily, seasonally, and yearly because of the timing of the pesticide application, the vulnerability of the water supply to pesticide loading through runoff, spray drift and/or leaching, and changes in the weather. Concentrations are also affected by the method of application, the location, and characteristics of the sites where a pesticide is used, the climate, and the type and degree of pest pressure, which influences the application timing, rate used, and number of treatments in a crop production cycle.
EPA may conduct a drinking water assessment DWA for a national scale depending on the pesticide use under evaluation. A national scale DWA may use a single upper-end pesticide concentration as a starting point for assessing whether additional refinements are needed or estimated pesticide concentrations for certain sitespecific scenarios that are associated with locations in the United States vulnerable to pesticide contamination based on pesticide use patterns. Ref. 24
at 22.
EPA may also conduct a regional scale DWA to focus on areas where pesticide concentrations may be higher than the DWLOC. Under this assessment, EPA
estimates pesticide concentrations across different regions in the United States that are subdivided into different areas called hydrologic units HUCs.
There are 21 HUC 2 regions with 18 in the contiguous United States. These areas contain either the drainage area of a major river or a combined drainage of a series of rivers. This information can eb found at: https water.usgs.gov/GIS/
huc.html. Estimated pesticide concentrations under this approach would be associated with a vulnerable pesticide use area somewhere within the evaluated region. Ref. 24 at 23.

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